One form of digital printing that is analogous to conventional inkjet printing uses solid inkjet printing. Solid ink jet printers deposit or jet marking particles in a molten or liquid state onto a receiver to form an image. The image is then forced through a nip in a spreader formed by a plurality of rollers, at least one of which is heated to a temperature sufficient to melt the solid ink and subject the inked image to a pressure, while molten, to reduce the height of the ink and spread the ink. As the image exits the nip, the adhesive forces between the molten ink and the contacting roller extends the ink, resulting in variable gloss across the image and differential gloss at the edge of high density regions of the image. This is particularly objectionable in mid density regions especially those comprising halftone patterns. As used herein, the term solid ink refers to an ink that is in solid form at room temperature but is liquefied by heat and jetted when in the liquid or melted state. Solid ink jet ink is a phase change material that is either in a solid or liquid phase. In contrast to toner particles, which comprise an amorphous polymeric binder, these inks are crystalline. Amorphous polymers tend to be hard, whereas crystalline polymers are waxy. Amorphous polymers soften at their glass transition temperature. More specifically, the Young's modulus of an amorphous polymer decreases from about 3 GPa to about 3 MPA as the glass transition temperature Tg is traversed. This is not a phase transition, as there is not a plurality of thermodynamic phases. In general, the Tg of an amorphous polymer is fairly broad. Accordingly, the softening of an amorphous polymer occurs over a temperature range typically between 5 and 10° C. The toughness of amorphous polymers makes them highly suitable for printing applications. Nevertheless, because they require spreading, it is very difficult to provide anacceptable gloss that does not have significant variations.
In contrast to amorphous polymers, crystalline polymers melt at a melting temperature Tm. At this temperature, the polymer undergoes a rapid transition from a solid phase to a liquid phase. The phase transition is sharp, making crystalline polymers suitable for applications where sharp softening temperatures, such as solid inkjet applications, are required. However, because of the crystal structure, crystalline polymers are waxy. Solid ink jetted images have significant relief, with the amount of relief varying with the density of the print. This results in a waxy feel to the print. The variations in density give rise to gloss variations with density as well as gloss variations occurring at the edge of a high density area. The prints are also as subject to damage caused by abrasion. Moreover, these prints can also adhere to one another when they are stored together and placed under pressure due to the flow of the ink.
In US Patent Publication No. 20010102525, relief is addressed by subjecting the inkjet image to heat and pressure in a spreader. The spreader includes a spreader roller and a metal pressure roller opposing the spreader roller.
Oliophobic materials tend to be hydrophilic, i.e. they tend to have high surface energies. Low surface energy liquids tend to wet and adhere to high surface energy solids. Many crystalline polymers suitable for use as solid inkjet inks have low surface tensions, i.e. between approximately 25 dynes/cm and 40 dynes/cm. Most oliophobic surfaces have surface energies in excess of 40 ergs/cm2 and therefore should be wet by the molten ink. This can cause offset wherein some of the molten solid ink sticks to the metal pressure roller.
The present system also has limitations on controlling friction. This further limits the choice of suitable coating materials. In addition, subjecting the image to heat and pressure as described in the related art can result in the prints having a high gloss. This can be objectionable in itself. However, it also augments differential gloss.
Gloss control is difficult with solid ink jet inks such as those used in solid ink jetting. Specifically, solid ink jet inks such as those jetted in solid ink jetting devices, first melt the ink. The ink is then jetted, while in liquid form, and cools and resolidifies on the receiver. Melting is a first order phase transition and is characterized by the presence of a latent heat. According to the laws of thermodynamics, the latent heat is proportional to the change in volume that occurs during first order phase transitions. As a result of the sudden change in volume, internal stresses are frozen into the ink.
The ink droplets solidify in a manner that results in the droplets being in physical relief of the receiver. To reduce the relief, the images are subjected to heat and pressure by a series of heated rollers that remelt the ink and spread the images. This results in the surface of the ink having a glossy appearance as well as a loss of resolution and detail caused by the spreading of the ink and the casting of the inked images against the spreader rollers. The gloss is objectionable when its level is different from the underlying gloss of the receiver and visible in unprinted areas. Moreover, differential gloss can occur with image density variations as the gloss of the receiver is averaged with the gloss of the ink. This is especially noticeable at the edges of high and low density portions of a print.